[clangd] Support simpler JSON-RPC stream parsing for lit tests.

Summary:
Instead of content-length, we delimit messages with ---.
This also removes the need for (most) dos-formatted test files.

Reviewers: ioeric

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D42919

llvm-svn: 324333